Bay and Catchments

Report Card for July 2013 – June 2014

This Report Card provides a snapshot of water quality in the Bay and catchments from 1 July 2013 to 30 June 2014. The quality of the water is given an overall score by combining the results of a number of standard water quality parameters: nutrients, water clarity, dissolved oxygen, salinity, pH, metals and algae.

Summary Table: The table below shows the percent of the Bay and surrounding catchment in one of five ‘condition’ categories. See scoring method for more information.

Area

Score

17%

Very Good
Near-natural high quality waterways

Forested upper catchments and entrance to Port Phillip Bay

24%

Good
Meets Victorian water quality standards

Upper catchments and Port Phillip Bay

21%

Fair
Some evidence of stress

Mostly the main rivers and tributaries in the middle catchment areas on the urban fringe

23%

Poor
Under considerable stress

Mostly lower reaches of the main rivers and urban tributaries

15%

Very Poor
Under severe stress

Mostly small urban tributaries

Routine monthly monitoring across 110 sites accounts for 89 per cent of the Bay and catchments area. The remaining 11 per cent of unmonitored catchments are mostly rural and typically score as Fair for this region.

What do the results mean?

The overall water quality scores across the Bay and catchments vary greatly depending on local catchment use and development, but there is a general trend of declining water quality from the most upstream, ‘pristine’ areas, to the more urbanised downstream areas.

A good way to make sense of the results is to follow the flow of water down the catchments into Port Phillip Bay.

The upper areas of the Port Phillip Bay catchment are well forested, and the waterways tend to have ‘Good’ to ‘Very Good’ water quality because there is very little development that could impact on water quality.

Moving downstream, the waterways flow though rural areas. Here, vegetation-clearing around waterways, sediment runoff and use of agricultural chemicals can impact the waterways and reduce water quality. The water quality in these rural areas is usually ‘Fair’.

Towards the bottom of the catchment, waterways flow through a highly urbanised catchment before entering the bay. Water quality in these areas is often ‘Poor’ to ‘Very Poor’ due to the impacts of residential development and industry. Pollutants in urban areas include:

• oils and hydrocarbons (fuels) from vehicles,

• zinc from rubber tyres,

• metals from industrial estates,

• soil from construction sites,

• nutrients from garden fertilisers

• litter from our streets

Port Phillip Bay receives water from the rivers and drains across the Bellarine, Werribee, Maribyrnong, Yarra, Dandenong and Mornington catchments. Pollutants in this water have an impact on the Bay’s water quality where it enters. Areas of the Bay that are closer to mouths of rivers and drains have poorer water quality more often than areas that are further away.

We also know that weather can have a big impact on water quality, so it helps to look at what the weather was like over the reporting period for this Report Card. Between 1 July 2013 and 30 June 2014 the Port Phillip Bay region received slightly below-average (9 per cent below average) rainfall and it was very dry during summer (rainfall approximately 50 per cent below average) (source: Bureau of Meteorology, Melbourne Regional Office). These slightly dry conditions generally make for fairly stable water quality. So if we see any big changes from previous years, it’s likely due to a factor other than the weather.

Yarra River, Melbourne CBD. Source: Parks Victoria

Changes over Time

Since the 1970s the Bureau of Meteorology has identified a long-term ‘drying’ trend in the Melbourne metropolitan region, which shows that the annual rainfall is decreasing by 50 mm every 10 years. This is one of the most significant rainfall declines across the state of Victoria.

Since 2000, we’ve seen the climate change from long-term drought (1998-2009), and move to generally wetter conditions thereafter. Increased flows in recent years have decreased salinity and increased dissolved oxygen in the waterways, which has generally resulted in improvements to water quality. These increased flows have also transported more pollutants such as sediment and nutrients to the Bay, but despite this, Bay water quality is also improving. These improvements are likely due to a combination of factors including relatively moderate rainfall (i.e. not as low as drought levels but not as high as in 2010/11) and a reduction in the amount of nutrients sewerage treatment plants discharge into the Bay.

Notably, the Dandenong and Mornington catchments have consistently shown ‘Poor’ to ‘Very Poor’ water quality regardless of improvements to water quality in other regions. In these catchments, waterways tend to be small and highly modified from their previous natural state, making them more susceptible to the impacts of urban and industrial land use and the pollution that these land uses bring.

Click on the map above for further details about water quality in each catchment.